| mask | 1. A cover, or partial cover, for the face, used for disguise or protection; as, a dancer's mask; a fencer's mask; a ball player's mask. 2. That which disguises; a pretext or subterfuge. 3. A festive entertainment of dancing or other diversions, where all wear masks; a masquerade; hence, a revel; a frolic; a delusive show. "This thought might lead me through the world's vain mask." (Milton) 4. A dramatic performance, formerly in vogue, in which the actors wore masks and represented mythical or allegorical characters. 5. A grotesque head or face, used to adorn keystones and other prominent parts, to spout water in fountains, and the like; called also mascaron. 6. In a permanent fortification, a redoubt which protects the caponiere. A screen for a battery. 7. <zoology> The lower lip of the larva of a dragon fly, modified so as to form a prehensile organ. Mask house, a house for masquerades. Origin: F. Masque, LL. Masca, mascha, mascus; cf. Sp. & Pg. Mascara, It. Maschera; all fr. Ar. Maskharat buffoon, fool, pleasantry, anything ridiculous or mirthful, fr. Sakhira to ridicule, to laugh at. Cf. Masque, Masquerade. Source: Websters Dictionary (01 Mar 1998) |
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| mask of pregnancy | <medicine> A dark discolouration of the skin, usually local; as, Addison's melasma, or Addison's disease. Melasmic. Origin: NL, fr. Gr. Black spot. Source: Websters Dictionary (01 Mar 1998) |
| mask shell | <zoology> Any spiral marine shell of the genus Persona, having a curiously twisted aperture. Source: Websters Dictionary (01 Mar 1998) |
| Hutchinson's mask | The sensation experienced in tabetic neurosyphilis as if the face were covered with a mask or with cobwebs. (05 Mar 2000) |
| shadow mask | <microscopy> Also Aperture mask. In colour video monitors, a finely perforated or striped metal plate that is located between the three electron guns and the phosphor screen. The mask ensures that the electron beams carrying the R, G, and B signals land on the corresponding phosphors at every point in the picture. (05 Aug 1998) |
| nonrebreathing mask | A mask fitted with both an inhalation valve and an exhalation valve so that all exhaled gas is vented to the external atmosphere and inhaled gas comes only from a reservoir connected to the mask. (05 Mar 2000) |
| ecchymotic mask | A dusky discoloration of the head and neck occurring when the trunk has been subjected to sudden and extreme compression, as in traumatic asphyxia. (05 Mar 2000) |
| tropical mask | A bronze-coloured pigmentation, probably produced by hormone imbalance, occurring in gradually increasing areas on the face, neck, and chest in persons exposed continuously to the tropical sun; similar to chloasma of the temperate zone, but intensified because of strong sunlight. Synonym: tropical mask. (05 Mar 2000) |
| laryngeal mask | A tubular oropharyngeal airway with an inflatable rim at the distal end that when inflated creates an airtight seal immediately above the larynx. (05 Mar 2000) |
| luetic mask | A dirty brownish yellow pigmentation, blotchy in character, resembling that of chloasma, occurring on the forehead, temples, and sometimes the cheeks in patients with tertiary syphilis. (05 Mar 2000) |
| alactic oxygen debt | That part of the oxygen debt that is not lactacid oxygen debt; during recovery, stores of ATP and creatine phosphate must be replenished by oxidative metabolism, and a small amount of oxygen is also needed to restore the normal oxyhemoglobin levels throughout the circulating blood. (05 Mar 2000) |
| alveolar-arterial oxygen difference | The difference or gradient between the partial pressure of oxygen in the alveolar spaces and the arterial blood: P(A-a)02. Normally in young adults this value is less than 20 mm Hg. See: alveolar gas equation. (05 Mar 2000) |
| arteriovenous oxygen difference | <physiology> The difference in the oxygen content (in ml per 100 ml blood) between arterial and venous blood. (05 Mar 2000) |
| biochemical oxygen demand | The amount of oxygen aerobicorganisms need to carry out oxidative metabolism in watercontaining organic matter, such as sewage. (09 Oct 1997) |
| carbon-oxygen ligases | <enzyme> Enzymes that catalyze the joining of two molecules by the formation of a carbon-oxygen bond. Registry number: EC 6.1 (12 Dec 1998) |